I'd like to make life easy and safe for the builders and install the stairs as early as possible.

I'd like to delay installing the stairs as late as possible so that I can preserve the nice oak wood treads and stringers - perhaps also having the plaster work in place so the stairs can be set flush to the wall.

Obviously there is a conflict - I can't have both.

I could buy cheep stairs and replace with the good ones later - but If I don't do this... how long is it reasonable for the builders to have to use a ladder?

I'd certainly leave the oak stairs until after plastering. They'll be used to using ladders, but scaffolding companies can also install a temporary staircase using treads such as these : http://www.apolloscaffoldservices.co.uk/steps.html or by installing an access tower in the stairwell.